Among organic materials, polyimides are polymeric materials that have the highest levels of heat resistance, chemical resistance and electrical insulating properties. In the electrical and electronic industry, for example, “KAPTON (registered trademark)” manufactured by DuPont, which is synthesized from pyromellitic dianhydride (PMDA) and 4,4′-diaminodiphenyl ether (pDADE), and “UPILEX (registered trademark)” manufactured by Ube Industries, Ltd., which is synthesized from biphenyltetracarboxylic dianhydride (BPDA) and p-phenylene diamine (pPD), are widely used as heat-resistant insulating materials. However, polyimides have a drawback of being hardly soluble in organic solvents as an adverse effect of having excellent chemical resistance. Thus, polyimides have poor processability and are mainly distributed in film forms.
Polyimide films are produced by dissolving an acid dianhydride having two acid anhydride groups in the molecule and a diamine having two amino groups in the molecule in a solvent to synthesize a polyimide precursor varnish called “polyamic acid”, coating and drying this precursor varnish, and then heating the resultant at about 350° C. It has been strongly desired to handle polyimides in a solution state, and a number of developments have been made in solvent-soluble polyimides.
However, in order to obtain a solvent-soluble polyimide, it is generally required to use monomers having high solubility, that is, low heat resistance; therefore, the resulting polyimide has a low heat resistance and a low chemical resistance. Meanwhile, as a method of handling a polyimide as a solution without sacrificing heat resistance and chemical resistance, there is a method in which a user forms a coating film with a polyamic acid solution and subsequently performing imidization. However, since not only the handling and storage thereof are difficult because polyamic acid solution is easily affected by the humidity but also a heat treatment at about 350° C. is required for imidization of polyamic acid, the use of a polyamic acid solution is limited to coating on heat-resistant materials. Under such circumstances, examples of a technology relating to a solvent-soluble polyimide include those disclosed in Patent Documents 1 and 2.